It is known to provide data encoding pattern on products, for example documents, so that a hand held device, such as a pen, can read the data encoded in the pattern and use it, for example, to detect its position as it is moved over the document. Where the hand held device is a pen arranged to mark the product, the position of marks made on the documents can be detected by the pen, thereby enabling the position of the marks on the document to be stored electronically as they are made.
Where the product has human readable content applied to it as well as the position identifying pattern, it can be a problem to ensure that the pattern is not obscured by the content. It is possible to overcome this problem by printing the pattern and the content using different inks. This enables the pen to detect the pattern using light at a wavelength that is absorbed or reflected by the pattern but not the content, or by the content and not the pattern. However, this requires the use of two different inks, which can add to the complexity of the printing process.